Japanese Laid-open Patent Publication No. Hei. 11-166600, U.S. Pat. Nos. 6,142,902, 6,334,829, 6,419,604 and 6,440,022, and Japanese Utility Model Publication No. 2553206 all show double-sided engagement type silent timing chains in which link teeth protruding on the inside of a loop formed by the chain engage one or more sprockets while link teeth protruding on the outside side of the loop engage another sprocket so that the inside and outside sprockets rotate in opposite directions.
In a typical engine timing drive, the crankshaft sprocket, and the intake valve and exhaust valve cam sprockets are engaged with teeth on the inside of the loop and rotate in the same direction, while one or more auxiliary sprockets, for example an oil pump driving sprocket, are engaged with teeth on the outside of the loop, and rotate in the opposite direction.
FIGS. 6 and 7 show a conventional double-sided engagement type silent chain 500. The chain is composed of interleaved, alternate, first and second widthwise rows of link plates. A first row consists of inner link plates 510a flanked by a pair of outer link plates 520a. A second row consists of inner link plates 510b flanked by a pair of outer link plates 520b. Link plates 520b are the outermost link plates of the chain. The alternate rows are connected by connecting pins 530, which are fixed in pin holes 512 in the outermost link plates, 520b. The pin-holes in the other link plates, including outer link plates 520a as well as link plates 510a and 510b, fit rotatably on the connecting pins 530 so that the alternating rows of link plates can articulate.
Each link plate has a pair of teeth, the inner link plates 510a and 510b having teeth 511, which protrude in a first direction. The outer link plates 520a and 520b have teeth 521, which protrude in a second direction opposite to the direction in which the teeth 511 of the inner link plates protrude. Thus, when the chain is in the form of an endless loop, in the central part of the chain widthwise, the inner link plates 510a and 510b have teeth 511 protruding toward the inside of the loop, while along both sides of the chain, the outer link plates 520a and 520b have teeth 521 protruding toward the outside of the loop.
The back surfaces 513 of the outer link plates are typically flat, and co-planar in any straight portion of the chain. Likewise, the back surfaces 523 of the inner link plates are typically flat, and coplanar in any straight portion of the chain.
FIG. 8 shows link teeth 511 of the inner link plates 510a and 510b protruding toward the inside of a loop formed by chain 500 and in meshing engagement with sprocket teeth St of a crankshaft sprocket S on a crankshaft R. This double-sided engagement-type chain meshes with the crankshaft sprocket in the same manner in which a conventional single-sided engagement-type chain meshes with a sprocket. The inwardly projecting teeth of the chain come into meshing engagement with camshaft sprockets in a similar manner.
FIGS. 9 and 10 show how teeth 521 of outer link plates 520a and 520b engage sprocket teeth St of a sprocket S, on a shaft R, and disposed on the outside of the loop formed by the chain. As shown in FIG. 11, the sprocket has two sets of teeth, the two sets being axially spaced from each other by a distance such that they can mesh with the teeth of the outer link plates 520a and 520b. 
When the link teeth 521 of the outer link plates 520a and 520b engage with the sprocket, the inner link plates 510b come into sliding contact with the inner wall surfaces of the sprocket teeth St. Contact between the inner link plates 510b and the inner wall surfaces of the sprocket teeth prevents lateral wobbling movement of the chain.
As explained above, in the conventional double-sided engagement type silent chain 500, a connecting pin 530 is fitted to the pin hole 512 of the outermost side link plate 520b. The stress applied to the inner circumferential surface of the pin hole 512 in the outermost link plate 520b when outer link plates 520a and 520b both engage a sprocket, and the tensile stress generated at the inner circumferential surface of the pin hole 512 when the connecting pin 530 is fitted to the pin hole, act synergistically to reduce the strength of the outermost link plate 520b. 
Since, in the conventional double-sided engagement-type silent chain, the inner link plates 510a and 510b and the outer link plates 520a and 520b have the same shape, the central portions of links, which extend from the tooth gap bottoms to the back surfaces of the link plates are poor in strength. In particularly, when a load is applied to an outermost link plate 520b, that is, a link plate to which a connecting pin 530 is fitted, cracks tend to occur in the vicinity of the tooth gap bottoms, causing a progressive decrease in the strength of the chain, and a reduction of the rotational fatigue strength of the chain.
Furthermore, although link teeth 521 are formed on the outermost link plates 520b, guide plates having no link teeth can be used instead. In such a case, increased wear can occur in the sprocket teeth where they contact the link teeth 521 of the outer link plates 520a adjacent the guide plates. Increased wear can also occur in the teeth 521. In order to suppress this wear, the plate thickness or the number of the outer link plates can be increased. However, these measures increase the overall width of the chain, which is disadvantageous because more space is required to accommodate the increased width of the chain.
When the shaft of an auxiliary device such as an oil pump or the like is driven by a sprocket engaged with the outer link plates 520a and 520b on the exterior of the chain loop, the number of the outer link plates 520a and 520b is ordinarily smaller than the number the inner link plates. As a result, bending and sliding loads exerted between the pin holes of the outer link plates 520a and the connecting pins 530 become large and cause increased contact wear between the pin holes of the outer link plate 520a and the connecting pins 530.
When the conventional double-sided engagement type silent chain 500 is in driven relationship with an engine crankshaft sprocket and in driving relation ship with an intake valve cam sprocket and an exhaust valve cam sprocket, the teeth of link plates 510a and 510b are disposed on the inner circumferential side of the chain loop and engageable with the sprocket teeth. Contact wear between the pin holes of these link plates 510a and 510b and the connecting pins 530 causes wear elongation of the chain.
An object of this invention is to address the above-mentioned problems by providing a double-sided engagement type silent chain in which the outermost link plates, into which the connecting pins are fitted, have improved impact resistance and fatigue strength in order to suppress the generation of cracks in their tooth gap bottoms when they collide with a sprocket, and in which the wear resistance of the inner link plates is also improved so that contact wear between the pin holes of the inner link plate and the connecting pins is reduced, and the resulting chain wear elongation is suppressed.